In order to adhere to the legal emission restrictions, modern motor vehicles with a spark ignition engine have a tank ventilation device by means of which fuel vapors escaping from the fuel tank are adsorbed in a fuel vapor reservoir (generally an activated charcoal container). Because the storage capacity of the fuel vapor reservoir is limited, it is necessary to regenerate said reservoir under suitable operating conditions of the internal combustion engine. For this purpose, the fuel vapor reservoir is connected to the inlet manifold of the internal combustion engine via a ventilation line. The controlled opening of a tank ventilation valve located in the ventilation line pneumatically connects the fuel vapor reservoir during the tank ventilation to the inlet manifold. Due to the partial pressure existing in the inlet manifold, the fuel vapors adsorbed in the fuel vapor reservoir are subsequently sucked as regeneration gas into the inlet manifold in order to partake in the combustion.
Through the hydrocarbons contained in the regeneration gas, in the event of the tank ventilation the composition of the fuel mixture undergoes a change. However, in order to ensure a sufficient combustion efficiency (quiet running) and an optimum exhaust gas purification it is necessary to adjust the air-to-fuel ratio of the fuel mixture to a defined value. In order to achieve this, the amount of metered fuel for the internal combustion engine is adjusted accordingly via injection valves. However, carrying out the tank ventilation is only possible in certain operating points of the internal combustion engine, namely at the point at which the partial pressure in the inlet manifold is high enough in order to suck in hydrocarbons that are adsorbed in the storage container. Therefore, the tank is preferably ventilated during idling or during partial load operation.
In addition, modern internal combustion engines have a lambda regulating device, i.e. a closed regulating loop for regulating the amount of fuel to be metered. Core elements of the lambda regulating device are represented by the lambda regulator, the fuel supply and a lambda sensor arranged in the exhaust gas tract of the internal combustion engine by means of which the composition of the exhaust gas can be measured and conclusions can be drawn with respect to the composition of the fuel mixture. In order to ensure optimum exhaust gas purification and good combustion stability, the exhaust gas composition or the fuel mixture composition has to be adjusted to a defined target value. In the case of a deviation of the composition of the fuel mixture or the exhaust gas from the target value, the amount of fuel supplied to the internal combustion engine is corrected as needed and in this way regulated to the target value.
The correction values determined in the process can also be used for the adjusting of the operating point-dependent pilot control of the components of the fuel injection and for diagnosing the same. For example, injection valves can have different characteristics depending on the operating point, which already have to be taken into consideration in the pilot control during an operating point changeover. In addition, the operating behavior of these components can also change over the service life. Such deviations in the behavior of these components are identified by the lambda regulating device and taken into consideration in the form of a corresponding correction value in the pilot control of the specific component. As a result, the combustion stability can be improved, which is particularly important during idling. The adjustment of the pilot control of the components of the amount of fuel regulating loop is also known as the adjustment of the basic mixture.
So far, it has not been possible to carry out the tank ventilation operation and the adjustment of the basic mixture simultaneously because it was not possible to separate the relevant effects from the fuel mixture composition. As already described above, the tank ventilation and the adjustment of the basic mixture are preferably carried out in operating ranges of the idle operation and the lower partial load operation of the internal combustion engine. By introducing new concepts for saving fuel such as for example the automatic stop-start mechanism, possible time windows for carrying out the above-mentioned processes are further reduced.